1. Task 10.5 High Precision THIRD HARMONIC SC Cavity Alignment/Diagnostics/BPM with HOM Measurements
Nicoleta Baboi, Ursula van Rienen
Roger M. Jones
DESY, Univ. of Rostock, Univ. of Manchester/ Cockcroft Inst.
EuCARD WP10.5 Task leader on HOM Distribution (inc. 3 sub-tasks)
EuCARD Board member

2. WP 10.5 Aspects of HOMs in SC Accelerator Cavities –EuCARD FP7
TASK 10.5
HOM Distribution
R.M. Jones
Sub-Task
Name
Coordinating Institute/Univ.
10.5.1
HOMBPM
DESY
10.5.2
HOMPM
10.5.3
HOMCD
Cockcroft/Univ. Manchester
10.5.4
HOMGD
Univ. Rostock
TASK 10.5
HOM Distribution
R.M. Jones
10.5.2
10.5.3
Four-year task due to staff resources commuted to Three years
I. Shinton, PDRA (assigned for a further year, until Nov. 2010)

3. in SC Accelerator Cavities -Staff
Task 10.5 HOM Diagnostics
in SC Accelerator Cavities -Staff
Sub-task leaders: Nicoleta Baboi (DESY), Ursula van Rienen (Univ. Rostock), Roger M. Jones (CI/Univ. Manchester).
PDRAs: Hans-Walter Glock (Univ. Rostock), Ian Shinton (CI/Univ. of Manchester), TBA (DESY)
Ph.Ds: Nawin Juntong (CI/Univ. Manchester), Chris Glasman (CI/Univ. Manchester) WP WP WP
H-W Glock, Univ. of Rostock, PDRA
U. Van Rienen, Univ. of Rostock
N. Baboi, DESY
C. Glasman, CI/Univ. of Manchester PhD student
(PT on FP7)
N. Juntong, CI/Univ. of Manchester PhD student
(PT on FP7)
I. Shinton, CI/Univ. of Manchester PDRA
P. Zhang, DESY/Univ. Of Manchester

4. 10.5 Overview of the Function of Third Harmonic Cavities
Fermilab has constructed a third harmonic accelerating (3.9GHz) superconducting module and cryostat for a new generation high brightness photo- injector.
This system will compensate the nonlinear distortion of the longitudinal phase space due to the RF curvature of the 1.3 GHz TESLA cavities prior to bunch compression.
The cryomodule, consisting of four 3.9GHz cavities, has been installed in the FLASH photoinjector downstream, of the first 1.3 GHz cryomodule (consisting of 8 cavities).
Four 3.9 GHz cavities will provide the energy modulation, ~20 MV, needed for compensation.

5. Illustrative energy (not to scale)
WP GHz Parameters
Illustrative energy (not to scale)
Number of Cavities 4
Active Length
0.346 meter
Gradient
14 MV/m
Phase
-179º
R/Q [=U2/(wW)]
750 Ω
Epeak/Eacc
2.26
Bpeak
(Eacc = 14 MV/m)
68 mT
Qext
1.3 X 106
BBU Limit for HOM, Q
<1 X 105
Total Energy
20 MeV
Beam Current
9 mA
Forward Power,
per cavity
9 kW
Coupler Power,
per coupler
45 kW
Adding harmonic ensures the 2nd derivative at the max is zero for total field (could use any of the harmonics in the expansion, but using the lowest freq. ensures the transverse wakefields ~ 3 are minimised).
The third harmonic system (3.9GHz) will compensate the nonlinear distortion of the longitudinal phase space due to cosine-like voltage curvature of 1.3 GHz cavities.
It will linearise the energy distribution upstream of the bunch compressor thus facilitating a small normalized emittance ~ m*rad.

6. bunch compressors
bypass line
gun
undulators
FEL beam
ACC39
4 cavities within
3.9GHz Module
collimator section
dump
5 accelerating modules
with 8 cavities each
1.3GHz SC, typically MeV, 1 nC charge for FLASH/XFEL
HOMs generated in accelerating cavities must be damped.
Monitored HOMs facilitate beam/cavity info
Forty cavities exist at FLASH. -Couplers/cables already exist.
-Electronics enable monitoring of HOMs (wideband and narrowband response).
Based on 1.3 GHz (SLAC/FNAL/DESY) Diagnostics – will be redesigned for ACC39 as part of EuCARD

7. WP 10.5 Response of HOM modes to beam

8. WP 10.5 Analysis of Narrowband Signals – Beam Position
Resolution of position measurement.
Predict the position at cavity 5 from the measurements at cavities 4 and 6.
Compare with the measured value.
X resolution
9 microns
Y resolution
4 microns

9. 10.5 Aspects of HOMs in 3.9 GHz SC Accelerator Cavities
HOM based Beam Position Monitors (HOMBPM)
Initial electronics have been developed for single bunch and installed at FLASH allowing the beam to be centered to ~ 5 m.
Method needs to be verified with additional modes
Multi-bunch issues need to be understood.
The 3.9 GHz bunch shaping cavities installed in FLASH can readily dilute the beam emittance –important to instrument with electronics modules to diagnose the beam position and improve the emittance.
HOM based Cavity Diagnostics (HOMCD)
Simulations conducted on complete spectrum (up to 6 bands)
Participated in characterisation, S21 of HOMs at DESY CMTF
HOM spectrum allows one to ascertain the cavity alignment and cell geometry.
In process of investigating:
-    mechanical deviations of individual cells from the ideal geometry,
-    cell-to-cell misalignment,
-    deformation of fields by couplers.
This requires beam-based measurements at FLASH/DESY
I. Shinton, CI/Univ. Manchester PDRA at FLASH (DESY) HOM shift
C. Glasman, CI/Univ. Manchester Ph.D. student at FLASH (DESY) HOM Shift

10. Shinton/Juntong, Oct’09 CMTF measurements
HOM based Geometrical Dependencies (HOMGD)
Combining finite element and S-matrix cascading techniques allows the eigenmodes in multiple accelerating cells and cavities to be efficiently modeled. The University of Rostock and the University of Manchester have developed a suite of codes.
Means to determine
Applying these powerful computing methods in order to specify allowable tolerances on fabrication and alignment of the TESLA cells and cavities (see Shinton et al., SRF 2009)
Manchester/Cockcroft Exp/Beam Time Shifts at FLASH (DESY)
Jan/Feb 2010 installation in FLASH anticipated and beam-based measurements of modes
Oct Shinton (PDRA)/Juntong (Ph.D. Student); CI/Univ. of Manchester Participated in S21 mode measurements of 4-cavity modules at CMTF (Cryo-Module Test Facility), DESY. Coupled modes observed. Data analyzed.
September Shinton (PDRA)/Glasman (Ph.D. Student), CI/Univ. of Manchester)
21/9/08 – 29/9/08: 1: Collaborative shift, 3 HOM phase/position assigned shifts
January 2008
15/1/08-23/1/08: 5 shifts in total: remote access control of the machine achieved, 5 collaborative shifts in which calibration data was taken, Multibunch data taken, Phase measurements taken across module 5 for various offsets (beam moved in a circle) – broadband data.
Shinton/Juntong, Oct’09
CMTF measurements

11. 10.5.2 HOMs in 3.9 GHz SC Accelerator Cavities
Dipole circuit model (double chain) for the 3.9GHz cavity
Monopole modes for 6 bands characterised with circuit model
Circuit model allowed 6 dipole bands to be characterised –dispersion curves illustrated to the right and corresponding R/Qs below, together with simulations performed with MAFIA code. 11

12. 10.5.2 HOMs in 3.9 GHz SC Accelerator Cavities
Cavity modes up to 10GHz –allows identification of potential trapped modes and modal types, monopole, dipole, quadrupole and sextupole
Contains all 6 cavity dipole bands below 10GHz
The HFSS results agree well with by MAFIA simulations
E-field distribution
ω/2π (GHz)
Band type
R/Q: /cm2
4.2953
D Band 1 #1 EE
0.00
4.3580
D Band 1 #2 EE
0.29
4.4460
D Band 1 #3 EE
4.5388
D Band 1 #4 EE
1.08
4.5972
D Band 1 #5 EE
0.79
4.6399
D Band 1 #6 EE
0.16

13. 10.5.3 HOMs in 3.9 GHz SC Accelerator Cavities
Simplified CAD model of single ACC39 cavity
S21
(dB)
Transmission
Spectra
/2 (GHz)
Mode spectra of a single cavity of ACC39 obtained from CST and mode cascading simulations.
Here the reference result, CSC-computations using tetrahedral the f-domain solver, hexahedral t-domain and eigenmode-based solver, are shown in blue, red, green and black, respectively.

14. Concluding Remarks on Task 10.5
Third harmonic cavities received by DESY and have been characterised at the CMTF.
HOM electronics will be built designed and tested for 3.9 GHZ cavities in 2010.
HOM simulations on cavity alignment/beam based alignment in progress for third harmonic cavities.
In addition to meetings during scheduled measurements at DESY, we have phone-in or EVO meetings, with slides on indico:

15. Task 10.5 Talks
Overview of HOM Distribution task, R.M. Jones (University of Manchester/Cockcroft Inst.)
HOMBPM Beam Position Monitors - planned and extant experiments, N. Baboi (DESY)
HOMCD Cavity Diagnostics, I. Shinton (University of Manchester)
HOMGD Geometric Dependencies, H.-W. Glock (University of Rostock)